(1) Field of the Invention
The present invention relates to esters of a poly-.gamma.-glutamic acid and a shaped body of a polymer composed mainly of esters of a poly-.gamma.-glutamic acid.
(2) Description of the Related Art
It is difficult to obtain a polymer having a high degree of polymerization, in which the carboxyl group at the .gamma.-position of glutamic acid is intermolecularly amido-bonded to the amino group at the .alpha.-position, by chemical synthetic means. However, the present inventors succeeded in advantageously preparing poly-.gamma.-glutamic acid (sometimes referred to as ".gamma.-PGA" hereinafter) by utilizing fermentative production by microorganisms. Accordingly, there is no hindrance to the supply of this polymer, which is expected to be applicable to various uses in view of inherent properties as a polyamide, that is, 4-nylon, and characteristics brought about by the carboxylate group at the .alpha.-position.
However, .gamma.-PGA is generally poor in the solubility in solvents and is soluble only in dimethyl sulfoxide (DMSO) and water only when .gamma.-PGA is an alkali metal salt (e.g. sodium or potassium salt). Furthermore, the melting point of .gamma.-PGA is high or the optimum melting temperature range of .gamma.-PGA is narrow, though the melting point or the optimum melting temperature range differs to some extent according to the polymerization degree. Accordingly, the melt-shaping method customarily adopted for polyamides is not easily applicable to .gamma.-PGA.
In order to obtain a film or fiber from .gamma.-PGA, .gamma.-PGA is once dissolved in a solvent and then, the polymer is shaped into a film or a fiber. Herein, formation of a yarn from a solution or molten liquid, that is, so-called "spinning", is included in "shaping". However, so far as the present inventors know, it is not easy to obtain a good film or fiber from .gamma.-PGA. In the case where DMSO or water is used as the solvent, only a solid, thick, brittle and opaque film is obtained, or spinning per se is difficult and formation of a fiber is not easy. As pointed out hereinbefore, the solubility of .gamma.-PGA in solvents other than DMSO and water, especially in organic solvents suitable for shaping, is generally poor, and it is difficult to improve the shapability of .gamma.-PGA into a film or fiber.